In conventional boilers, the flue gases are normally treated to maintain the pollutant content of the flue gases exhausted to the atmosphere below the environmental standards of the community. In industrial operation, the characteristics of the operation of the boiler are relatively uniform and the operating parameters of the boiler are established to assure that the pollutant level in the effluent flue gas meets or surpasses the community standard.
For efficient operation, the feed water to the boiler is normally preheated by the exhausted flue gases in order to economize by retrieval of the heat energy in the flue gases before they are discharged through the stack. There is a critical stack temperature which must be maintained for efficient operation. The stack temperature is normally about the saturation temperature or acid dew point temperature of the flue gas, typically below 270.degree. F. (130.degree. C.).
Preferably, the feed water in one or more economizers or heat exchangers reduces the temperature of the flue gases preferably close to their saturation temperature before they are discharged.
The acid content of the flue gases is reduced by directing the gases to a scrubber or other apparatus which serves as a reactor in which a lime slurry or other reactant in a liquid carrier is introduced into the gas stream to remove the acid pollutants in the flue gas. In the reactor, the gas flow evaporates the moisture from the slurry as the reactant reacts with the acid pollutants, is dried and is separated out of the gas flow. Any residual reactant which is not separated out of the gas flow is entrained in the gas flow which carries the entrained material to a separator which separates out the particulate material from the gases. A typical separator consists of a baghouse filter in which fabric bags serve as the filter medium in which the effluent gas is caused to flow through the bags and deposit the entrained material on the bag surface. In order to avoid premature blockage of the bag surfaces, the entrained material should be free from moisture so as to avoid caking of the material on the bag surfaces.
When burning standard fuels, the boiler, the economizers, the reactor and the filter may be set to accommodate their operation to the particular characteristics of the fuel and the effluent generated in the boiler by the fuel. For example, in the reactor, the composition of the slurry is selected to assure a stochiometric ratio above 1.0 between the reactant and the acid components of the particular flue gas, and to assure the proper quantity of liquid carrier to afford complete evaporation of the carrier by the flue gas which should be discharged at the desired stack temperature. During start-up and shutdown, the boiler conditions depart from the optimum and there is a substantial loss in efficiency, but such losses are tolerated because of the infrequency of their occurrence.
When burning municipal solid waste in a boiler, however, the composition of the fuel may vary considerably with the source of the waste material being burned, and the efficiency of the facility may be substantially less than the maximum in order to accommodate the variations in the characteristics of the material which is burned in the boiler. For example, pollutant concentration in the flue gas at the boiler may be as high as 250 ppm SO.sub.2, 500 ppm HCl and 50 ppm HF. 80% to 90% of these pollutants must be removed to meet community standards.